avformat/mpegts: resync from the smallest packet size on

This avoids the loss of a packet in many cases if the packet size was wrong.
It also improves detection of packet size changes

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mpegts.c

@@ -1974,7 +1974,7 @@ static int read_packet(AVFormatContext *s, uint8_t *buf, int raw_packet_size, co
         /* check packet sync byte */
         if ((*data)[0] != 0x47) {
             /* find a new packet start */
-            avio_seek(pb, -TS_PACKET_SIZE, SEEK_CUR);
+            avio_seek(pb, -raw_packet_size, SEEK_CUR);
             if (mpegts_resync(s) < 0)
                 return AVERROR(EAGAIN);
             else